We used to have a nice record player.
It was a combination turntable/cassette/radio unit, that I got around 1981.
Nine years later, we sold it for $10 at a garage sale. It was working fine
at the time; we simply hadnít used it, not since we got a little CD/cassette/radio
unit, which did the same thing, in half the space.

And now it is the year 2000.

At a local flea market, I recently
came upon an LP record that I dearly remembered from 3rd grade. Since I
had also bought a CD-RW drive, I thought it would be a simple matter to
transfer the record to CD. All I had to do was ask to borrow someoneís
record player. That was when I discovered the awful truth. There are hardly
any record players left. All the people I knew had disposed of their turntables
long ago. I thought I might pick one up cheap at a flea market or thrift
store. Not a chance. These sources were cleaned out, too, probably by guys
like me who wanted to do the exact same thing. At Goodwill, there were
a few record players, but usually missing the needles, or otherwise broken
in some serious way.

I finally found a turntable at an electronics
swap meet. It was a good one - a discrete component in its day - direct
drive, quartz-locked. And amazingly, it was $10. It was like finding gold!

When I brought the unit home, and tried
to use it, I got a few surprises.

I couldnít connect it. Old turntables
have RCA phone jacks, but modern stereos and computers do not. I was able
to purchase a little adapter separately at Radio Shack.

When I did get the turntable hooked
up, the volume was VERY soft. I got a rapid education on how turntables
work. Thereís no amplifier inside. The output level is just a few millivolts,
as opposed to the 0.7 volts or more of signal that the LINE_IN of a computer
expects, or VIDEO_IN of a stereo. If the stereo had a PHONO input on it,
that would be just the thing, but itís no surprise that the feature has
been deleted from present-day systems. My computer had a MIC input, which
provided extra amplification, but that was only mono.

There was no bass. My jerry-rigged
solution was to use my amplified computer speakers as a stereo preamp,
then run the headphone outputs from the speakers into the LINE_IN of the
computer. This gave me enough volume, but the sound was cheap and tinny.
I knew that the turntable was capable of better than that. I went back
to the web to do some reading, and learned about RIAA equalization. Cutting
the master for a vinyl record was a mechanical process, and as such, the
tool didnít cut as deeply at high frequencies. To compensate, the recording
industry decided to electronically emphasize the high frequencies more
than the low ones, so that the grooves on a record would be the same depth
for equal volume. To play back a record, you need a preamplifier with a
reverse characteristic (one that emphasizes the low frequencies).

I saw a bunch of circuit suggestions in
application notes, and on the web, but they generally needed dual power
supplies (i.e., ±
12 volts). Since I wanted to embed the preamp into the turntable, my solution
had to run off of the single +22 volt internal supply.

I chose the LM324 op amp because it's
designed to run off a single supply, but mainly because it's cheap and
ubiquitous. There are better op amps made, but the sound quality from this
circuit was fine to my untrained ear. Almost any op amp will serve for
this circuit, even the dual-supply type.

Theory of Operation

The LM324, as a single supply device,
cannot handle a signal that goes below ground. The divider formed by R1
and R2 set the operating point of the amplifier (that is, the voltage with
no signal coming in) at about 2 volts. But why not set it at half the supply
voltage? This would be possible with two 100k resistors, but the amplifier
would take longer to warm up, since C4 would have to charge up to 11 volts
instead of just 2. Also, since theoretically, C4 never has more than about
2 volts across it, a cheaper and more compact capacitor can be used.

The input impedance of this type of
amplifier should be about 50k ohms, so the combination of R1 and R2 in
parallel is in that ballpark.

This circuit should work with any power
supply from about 3 volts to 32 volts. Youíll need to adjust R1 and R2
to get the right operating point, while maintaining 50k input impedance.
If you opt for a supply voltage on the low end, you will have to be especially
careful in setting the operating point and gain, to avoid having the output
clip to ground.

The typical magnetic phono cartridge
will provide an input signal that swings a few millivolts above and below
ground. Input coupling capacitor C1 couples the signal up to the operating
point. Since this capacitor also never has more than about 2 volts across
it, a multilayer ceramic cap can be used, with a 6-volt rating. If you're
concerned about compactness, these capacitors come in the 0805 surface
mount form factor nowadays. In fact, this whole circuit can be made thumbnail-size
by employing surface-mount components.

As with any op amp, the gain of the
first stage is equal to the impedance of the network between the output
and the "-" input, divided by the impedance of the network between the
"-" input and ground. In other words,

Gain = [Impedance of R3, R5, C2, C3]
/ [Impedance of R4, C4]

At "medium" frequencies, consider C4
to be a short, and C2 to be open. This gives a gain of R3/R4, which is
470k / 1.5k, or about 300. But, in fact, the capacitors are neither quite
open nor shorted, so the actual gain is about 100 (20 dB).

C4 has a significant impedance at very
low frequencies, and serves to reduce the gain of the amplifier at rumble
frequencies. C4 is not strictly necessary, and R4 could be tied directly
to ground.

R3, R5, C2, and C3 together have an
impedance that reduces itself at higher frequencies. This forms the basis
of the RIAA compensation - high frequencies are rolled off, creating an
effective "bass boost".

To adjust the gain of the amplifier
without sacrificing the RIAA equalization response, scale both R1 and C4
together as a set, and R3, R5, C2, and C3 together as a set, remembering
that a capacitor's impedance is inversely proportional to its capacitance,
at any given frequency. For instance, if you needed to halve the gain,
you could double R4 to 3k (2.7k or 3.3k are practical values), and halve
C4 to 11 uF (10 uF). Or, you could halve both R3 and R5, while doubling
C2 and C3. If you wanted to double the gain, you would make the opposite
adjustments. I wouldn't advise increasing R3 to more than 1 megohm, so
that signals will remain large compared to noise.

Ideally, you would like the output
of the circuit to be about 1 volt peak-to-peak during a loud section of
music. If the output of your phono cartridge is large, then the output
of this circuit may be so large as to clip against ground. You can see
this if you have an oscilloscope on hand, but if, like me, you don't, you
can run the output into your computer's sound card, make a recording, and
examine the resultant output file (the .wav file) graphically with a player
such as the freeware utility Wave Splitter. If you notice that the positive
samples go all the way up to 30000, but the negative ones seem strictly
limited to (say) -20000, the amplifier is clipping, and the gain will need
to be reduced as explained in the preceding paragraph.

The second op amp is configured as
a unity-gain voltage follower. It's not strictly necessary, but I thought
the whole thing sounded better when the primary stage wasn't loaded with
whatever was connected at the output. (Thereís no cost penalty, because
the LM324 packs 4 op amps into one package.) This second stage could be
used for gain, too, I suppose, but it wasn't necessary for me.

Capacitor C5 couples the output to
the line input of your computer sound card, the AUX input of your stereo,
or headphones. In the last case, C5 prevents the 2 volt DC operating voltage
from straining the headphones. The bass response of cheap headphones is
lousy, anyway, so if that's what you're using, the capacitor can be made
smaller. If you're only connecting to a PC or stereo, you can probably
get away without C5, since these devices generally have their own coupling
capacitors embedded. Note that when the preamp is first powered up, there
will be a voltage spike across C5, theoretically equal to the supply voltage,
so choose a voltage rating accordingly.

C6 filters the incoming power. If the
incoming power is particularly noisy, you could put a 100 ohm resistor
between positive power and C6, then power the rest of the circuit off of
the junction point of this resistor and C6. I chose 100 uF at 35 volts
for C6, but only because I had one lying around.

None of the resistors in this design
should dissipate more than 1/10 watt.

Only one channel is shown in the schematic.
If you want stereo, you'll need to wire a second channel the same way.
Fortunately, the LM324 has enough individual sections to build two channels.

Figure 2 - The first breadboard
had only one channel.

If you decide to build one of these
for yourself, I recommend that you wire only one channel of stereo, first.
This will provide a good debug opportunity. If you're like me, and don't
have an oscilloscope at your disposal at home, bringup can be accomplished
with a simple voltmeter. Look for the 2.0 volt operating voltage at the
each of the inputs to both op amp sections. If it's not there, you'll need
to debug it. When I built my preamp, I had several problems, all of which
were traceable to solder splash. Be careful when you solder, and use good
lighting. It wouldn't hurt to do a detailed visual inspection of your circuit
before powering it on.

Figure 3 - Opening the unit
is a mess!

Figure 4 - Final wiring.

To my good fortune, this turntable
already had a well-regulated 22-volt power supply inside, which I was able
to tap for a few milliamps.

In place of the original RCA phono
plug cable, I ran a thin shielded cable to the input of the RIAA preamp.
A discarded CD-audio cable (the kind that connects a CD-ROM drive to a
computerís sound card) worked beautifully. Another thin cable connected
the output to a 3.5 mm stereo mini-plug.

Figure 5 - The only visible difference.

When the whole thing was finished,
the only visible difference was the little phone jack on the front of the
turntable. But it works like a charm!